Shoot-thru transducer FYI.
You can use any transducer that has a 83/200 kHz crystal, SI, DI, or 2D trolling motor or transom version. The 2D side of the Y cable just has the circuit for 2D sonar.
There has been few Side Imaging transducer failures recently reported on Bass Boat Central where one side has stopped working. You can use one of those for shoot-thru 2D.
With a Motor Guide Humminbird adapter, you can use a Lowrance trolling motor puck or transom transducer also.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, 360, RC-1